Cow's-milk protein allergy — branching viva

Opening

Examiner: A four-month-old, exclusively formula-fed infant presents with a two-week history of blood and mucus in the stools, distress on feeds, and an eczema flare. The mother wants to switch to a goat's-milk or A2 formula. How do you frame this? ^[1]

Candidate: I would frame this as suspected cow's-milk protein allergy, most likely the delayed non-IgE-mediated phenotype given the bloody mucoid stools, feeding distress and eczema, while first clearing the anaphylaxis and red-flag gate — faltering growth, dehydration, signs of FPIES or proctocolitis with systemic compromise. I would take a focused allergy and feeding history, examine growth and hydration, and confirm the diagnosis clinically by eliminating cow's-milk protein for two to four weeks with a planned rechallenge, rather than switching to a goat's-milk or A2 formula, which are not appropriate alternatives because their proteins are broadly cross-reactive. ^{[1] [2]}

Branch 1 — phenotype and the anaphylaxis gate

Examiner: Walk me through the two phenotypes and why the distinction matters. ^[1]

Candidate: There are two phenotypes and the distinction drives the assessment and management. IgE-mediated disease is immediate, within minutes to two hours, driven by specific IgE to cow's-milk proteins, and presents with urticaria, angioedema, vomiting, and the one I must not miss — anaphylaxis. Non-IgE-mediated disease is delayed, hours to days, cell-mediated, and presents with gastrointestinal and dermatological symptoms — blood and mucus in the stools, reflux, distress on feeding, faltering growth, and eczema. The distinction matters because IgE-mediated disease can be anaphylactic and is confirmed with skin-prick testing and specific IgE, while non-IgE-mediated disease is confirmed by elimination and rechallenge and is not detected on allergy testing. ^{[1] [2]}